Al-Matalib al-'Aliyah 587 — Tahajjud

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that he related: I mentioned qiyam al-layl (voluntary night worship), and some of those present said that the Messenger of Allah (peace and blessings be upon him) indeed said: "Half of it (the night), a quarter of it, or (at the least) the brief interval between two squeezes of a she-camel's udder (even if only that, but one should certainly stand in prayer)."
Imam Abu Ya'la recorded this hadith in his Musnad, and the narrators of its chain are thiqat (trustworthy); however, since Makhramah bin Bukayr's hearing from his father is not established, this narration is in the ruling of munqati' (broken) according to the hadith scholars, though other supporting reports concerning the encouragement of qiyam al-layl strengthen it.